FIG. 1 is a cross-sectional view of a conventional semiconductor photodetector device such as shown, for example, in "HIKARI TSUSHIN SOSHI KOGAKU", by Hiroo Yonetsu, published by Kogaku-Tosho Kabushiki Kaisha, page 372, in FIG. 6.7(c). This conventional photodetector device is now described in terms of a PIN photodiode having a P.sup.+ -type diffusion region/N.sup.- -type InGaAs/N-type InP structure.
In FIG. 1, a first conductivity type semiconductor substrate 2 comprises an N.sup.+ -type InP semiconductor layer 3 and an N-type InP buffer layer 4 disposed on the layer 3. An electrode 1 on the N side of the PIN structure (hereinafter referred to as N-electrode) is disposed on one of the major surfaces of the semiconductor substrate 2, and an N.sup.- -type InGaAs light-absorptive layer 5 is disposed on the other major surface. A light-transmissive layer 6 of N.sup.- -type InGaAsP is disposed on the light-absorptive layer 5. Alternatively, the light-transmissive layer 6 may be formed of N.sup.- -InP. A P.sup.+ -type diffusion region 7, which is a second conductivity type semiconductor region, is formed to extend through the light-transmissive layer 6 into the light-absorptive layer 5. A P-N junction 13 is formed between the P.sup.+ -type diffusion region 7 and the first conductivity type or N-type semiconductor which forms the light-absorptive layer 5 and the light-transmissive layer 6. The bottom portion 14 of the P-N junction 13 is located in the light-absorptive layer 5. A broken line 15 indicates the boundary of a depletion region 8 formed due to the presence of the P-N junction 13.
An Si.sub.3 N.sub.4 surface protection film 9, an anti-reflection film 10, and an electrode contact 11 are disposed on the surface of the light-transmissive layer 6, and an electrode 12 on the P side of the PIN structure (hereinafter referred to as P-electrode) is disposed on the surface protection layer 9 contacting the electrode contact 11.
Next, the operation of the conventional semiconductor photodetector device shown in FIG. 1 is explained.
FIG. 2 shows a circuit diagram in which 1 he semiconductor photodetector device of FIG. 1, i.e. a PIN photodiode 30, is used. In FIG. 2, h.nu. represents a photon incident on the photodiode 30.
The PIN photodiode 30 with the structure shown in FIG. 1 is typically used in such a circuit arrangement as shown in FIG. 2. The PIN photodiode 30 is connected through a resistor 31 to a voltage source 32 which applies a reverse bias to the photodiode 30.
Light which enters into the P.sup.+ -type diffusion region 7 (FIG. 1) of the PIN photodiode 30 through the anti-reflection layer 10 passes through the light-transmissive layer 6 and, then, is absorbed by the light-absorptive layer 5 to produce electron-hole pairs. Electrons and holes are separated by the electric field in the depletion region 8 so that electromotive force is generated between the N-electrode 1 and the P-electrode 12. The width of the depletion region 8 can be controlled by a bias voltage applied between the N-electrode 1 and the P-electrode 12. However, in the case of the conventional photodiode shown in FIG. 1 in which the depletion region 8 is located in the light-absorptive layer 5 whenever light is incident, an electromotive force is generated to provide light-induced current, with no bias voltage applied between the electrodes 1 and 12.
The depletion region 8 formed by the P-N junction 13 has a depletion region capacitance determined by the relative dielectric constant of the material used. The capacitance of the depletion region B affects the response speed of the photodiode 30. The response speed is dependent on the magnitude of a CR time constant, and the response speed decreases with an increase of the CR time constant. The capacitance C of the CR time constant is provided essentially by the capacitance C.sub.B of the depletion region 8 which is expressed by the formula (1). ##EQU1## In the formula (1), A represents the area of the P-N junction 13;
k represents the relative dielectric constant of the material of the layers 5 and 6; PA1 N.sub.B represents the carrier concentration in the layers 5 and 6; PA1 V.sub.b represents the bias voltage applied between the electrodes 1 and 12; PA1 V.sub.D represents a built-in voltage; and PA1 .epsilon..sub.o and q are the dielectric constant of a vacuum and the electronic charge, respectively.
The resistance R of the CR time constant is a load resistor for light-induced current (high frequency current) flowing through a coupling capacitor 34 between output terminals 35 and 36 and is equivalent to the value R.sub.L of a load resistor 33 shown in FIG. 2.
FIG. 3 is a cross-sectional view of another example of a conventional semiconductor photodetector device.
The device of FIG. 3 is different from the device shown in FIG. 1 only in that both the P.sup.+ -type diffusion region 7 and the depletion region 8 formed by the P-N junction 13 are located only within the light-transmissive layer 6. Now assume that the carrier concentrations N.sub.B of the light-transmissive layer 6 and the light-absorptive layer 5 are of the same order, and that the relative dielectric constant of the light-transmissive layer 6 is smaller than that of the light-absorptive layer 5. Then, as is understood from the formula (1), the device has a smaller depletion region capacitance and, hence, a higher response speed than a device such as the one shown in FIG. 1 in which the depletion region is formed to extend through the light-transmissive layer 6 into the light-absorptive layer 5. Needless to say, if the carrier concentration N.sub.B of the light-transmissive layer 6 is smaller than that of the light-absorptive layer 5, the depletion region capacitance is smaller and, accordingly, the response speed is much higher.
In this example, however, since the light-absorptive layer 5 does not include the depletion region 8, electron-hole pairs recombine without being separated into electrons and holes, so that no electromotive force is generated. Therefore, in order to obtain a light-induced current, it is necessary to apply a bias voltage sufficient to make the boundary 15 of the depletion region 8 extend into the light-absorptive layer 5.
Because of the above-described structures, conventional photodetector devices have problems including the following ones. When such a conventional semiconductor photodetector device is used in an optical communications system, it is coupled to an optical fiber device. In an arrangement where light from the end of the optical fiber is introduced directly into the light-incident surface of the semiconductor photodetector device, because the light beam diverges, the coupling between them is easier as the area of the light-incident surface of the photodetector device increases. A larger light receiving area or larger P-N junction area is also desirable for obtaining a larger light-induced current. However, as the P-N junction area increases, the depletion region associated with the P-N junction becomes larger accordingly. In FIG. 4, an equivalent circuit for the total depletion region capacitance C.sub.B of the conventional semiconductor photodetector device of FIG. 1 is shown in terms of depletion region capacitance C.sub.1 per unit area. As is seen from FIG. 4, with increase in area of the P-N junction, the number of depletion region capacitances C.sub.1 per unit area connected in parallel and, hence, the total depletion region capacitance C.sub.B increases. Accordingly, the CR time constant increases. This means that the response speed of the resulting semiconductor photodetector device is slow. Accordingly, it is desirable that the relative dielectric constant of the zone where there is a depletion region be as small as possible.
In the conventional semiconductor photodetector device of FIG. 3 since both the P.sup.+ -type diffusion region 7 and the depletion region 8 are located only within the light-transmissive layer 6, the depletion region capacitance is small and, accordingly, the response speed is high. However, during its operation a bias voltage must be applied so that the depletion region can extend into the light-absorptive layer 5. When a bias voltage is applied, leakage current caused by crystal defects and surface leakage current at the surface which the P-N junction intersects increase so that the S/N ratio for the output, the result of the light-induced current and the leakage currents, is degraded.